The use of contact lenses among the population is very widespread. In particular, there are known so-called “soft” contact lenses which, with respect to the other types of contact lens (rigid or semi-rigid), afford a greater level of comfort when worn by the user.
This advantageous characteristic is provided by the high hydrophilic capacities of the polymer materials of which the contact lenses are realized. Such polymer materials, being capable of retaining a relevant quantity of water, allows greater compatibility between the lens and eye, as well as a greater deformability of the lens which allows it to better adapt to the surface of the eye.
More specifically, soft contact lenses may be constructed from polymers which have a large number of hydrophilic functional groups, for example, hydroxyls, which are capable of forming weak dipolar bonds (hydrogen bonds) with the water molecules. As a result of these characteristics, soft contact lens can contain a proportion of water by weight of from 25% to 75% with respect to the total weight of the contact lens, forming a hydrogel in which a solid matrix, which defines the structural portion of the lens and is constituted by the polymer material, is immersed in a liquid component which is uniformly distributed in the solid matrix.
Known examples of polymer materials used in the construction of the solid component of soft contact lenses are constituted by HEMA-based polymers (hydroxyethyl methacrylate) or by silicone polymers.
In addition, the liquid component is typically formed by an aqueous solution of sodium chloride at 0.9% by weight, which is known as “physiological solution”, which reproduces, at least in terms of the sodium chloride content, the composition of the natural lachrymal film which superficially covers the cornea of the eye. In this manner, the contact lens is more compatible with the surrounding ocular environment and the presence thereof on the surface of the eye is less irritating for the user.
In fact, it is known that one of the greatest disadvantages arising from the use of contact lenses, including soft contact lenses, is represented by the sensation of ocular dryness which is caused by the reduction or by the breaking of the lachrymal film. This fact typically involves a sensation of irritation and intolerance which often compels the user to take off the lenses and which, over time, may also result in changes to the lachrymal functionality and to other pathological dysfunctions causing detriment of the eye (such as, for example, inflammations at the cornea or conjunctiva).
In order to overcome, at least partially, this disadvantage, it is known to use external solutions which are administered in drops directly to the eye, to which the contact lens is applied.
Those external solutions may be formed by simple physiological solutions, as defined above, or, in a more effective version, by aqueous solutions which are generally and generically known as “lachrymal substitutes”.
Those solutions have particular physical, chemical and rheological characteristics as well as biocompatibility characteristics which are capable of reproducing to the greatest possible extent the properties of the natural lachrymal fluid.
In particular, lachrymal substitutes have specific viscoelastic properties, in particular a prominent pseudo-plastic behaviour (with viscosity of the solution which decreases with an increase of the shear stress applied), as well as hydrophilic, muco-adhesive (in order to remain attached for the longest possible time to the mucinase component of the lachrymal film), mucomimetic (in order to simulate in the best manner possible the behaviour of the lachrymal film) and wettability properties, in addition, naturally, to general compatibility with the tissues of the eye.
Those particular properties are conferred on lachrymal substitutes by specific compounds, generally polymers, such as polysaccharides, polyvinyl alcohols, derivatives of cellulose, having a molecular weight which is relatively high, in the order of hundreds of thousands Dalton.
However, the beneficial action of those external solutions is limited over time, so that it may be necessary to carry out those operations several times per day, which operation is inconvenient and very unwelcome to the users.
A second technical solution to the problem mentioned above which is described in EP1861069, in the name of the Applicant, provides for the production of a soft contact lens in which a lachrymal substitute is incorporated therein.
In that case, the liquid component of the contact lens is not formed by a simple physiological solution but, instead, by a lachrymal substitute as defined above. This type of contact lens is obtained by immersing a dry semifinished product, which does not have any water at the inner side thereof and which is formed by the solid component of the contact lens, in an aqueous solution which also contains, in addition to 0.9% sodium chloride, an effective quantity, typically from 0.1% to 0.3% by weight, of an active compound which is capable of conferring on the solution adequate viscoelastic characteristics in addition to muco-adhesive and mucomimetic properties. In particular, that compound is preferably hyaluronic acid or a sodium salt thereof, or a galactoxyloglucan which is extracted from tamarind seeds (TSP), those polymers conferring on the aqueous solution the properties required in terms of muco-adhesion, muco-mimetic nature and pseudo-plasticity.
The contact lenses produced according to EP1861069, once they are applied to the eye, release a fraction of the lachrymal substitute contained therein, substantially improving the sensation of comfort of the user.
In this manner, however, the content of active compound which characterizes the liquid component of the contact lens, so as to allow the definition thereof as a lachrymal substitute, progressively tends to decrease, so that the contact lens, application after application, becomes less and less comfortable.
That loss over time of the excellent initial performance levels in terms of comfort, suggests the provision for contact lenses of this type of relatively frequent replacement, for example, weekly replacement or, even better, daily replacement.
In order to counteract the phenomenon of the loss of active compound of the liquid component of the contact lens in successive applications, EP1861069 suggests the use of a preserving solution, defined as a solution which is suitable for conserving the soft contact lenses between an application to the eye and the subsequent application (also known as “care solutions”), which is formed by physiological solution supplemented, in addition to a disinfectant agent, by an effective quantity of the active compound already present in the soft contact lens, for example, hyaluronic acid or TSP. In this manner, a fraction of active compound is absorbed in the external superficial layers of the contact lens, partially restoring the quantity of active compound previously released in the eye.
The efficacy of that expedient, however, is greatly limited by the fact that the absorption of the active compound within the contact lens is opposed by the difficulty which the polymers with a high molecular weight have in being introduced inside the lens, in particular when the lens is a lens which is already hydrated. In fact, the mean diameters of the pores of the solid component of the lens are in the order of magnitude of the molecules of active compound, so that the mere exchange by diffusion determined by the difference in concentration of active compound between the external solution with respect to the contact lens and the liquid component present inside the lens is generally insufficient to diffuse the polymer in a substantially uniform manner inside the contact lens.
Consequently, therefore, as anticipated above, the active compound is absorbed only at the external superficial layers of the lens, without extending into deep, in this manner preventing the introduction into the lens of additional polymer and therefore limiting the quantity of active compound incorporated in the contact lens.
In different application fields, it is further known to use a soft contact lens acting as a therapeutic carrier, in particular as a device which is capable of slowly releasing over time one or more medicaments at the surface of the eye. In this case, the aqueous solution comprising the medicament to be released in the eye is introduced in the body of the contact lens during the production step of the lens (at the reticulation step of the polymer which forms the body of the lens or also at the hydration step of the lens).
In that case, the simple immersion of the lens in an aqueous solution comprising the medicament is also not sufficiently effective to restore the medicament which is released by the contact lens in the eye.
Therefore, there exists a need to provide a method which allows the effective introduction into hydrated soft contact lenses of an active compound, in particular an active compound which is selected from a lubricating agent and a medicament to be released in a controlled manner inside the eye.